1. Field of the Invention
The present invention generally relates to the determination of fluid properties and, more particularly, to the determination of the thermal conductivity, thermal diffusivity, specific heat and fluid velocity of a fluid of interest.
2. Description of the Prior Art
A number of approaches have been devised to measure the thermal conductivity, thermal diffusivity, specific heat and fluid velocity of a fluid of interest. Typically, these and other properties are detected through the use of various types of detectors including resistive bridge type sensors.
One approach for determining thermal conductivity is described in U.S. Pat. No. 4,735,082 in which a heated element in one leg of a Wheatstone Bridge is placed or positioned in a cavity through which the sample fluid of interest is passed. The heated element is used to introduce a series of amounts of thermal energy into the fluid of interest at various levels by varying the input voltage via alternating current, voltage or power so that the changes in dissipated power caused by changes in the thermal properties of the surrounding fluid can be detected as voltage, current or resistance change signals. One of the measurable fluid properties is the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS. 1-5, very small and very accurate "microbridge" or "micromembrane" semiconductor chips supporting sensors have been described in which such microelements are used as heaters and sensors. Such sensors might include, for example, a pair of thin film sensor elements around a thin film heater element for measuring flow rates. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. No. 4,478,076, U.S. Pat. No. 4,478,077, U.S. Pat. No. 4,501,144, U.S. Pat. No. 4,651,564, and U.S. Pat. No. 4,683,159, all of common assignee with the present invention.
Another approach for measuring the thermal conductivity, thermal diffusivity and specific heat of a fluid is disclosed in U.S. Pat. No. 4,944,035 to Aagard et al. Aagard et al. discloses using a microbridge structure that has a heater film and at least one spaced sensor films. A pulse of electrical energy is applied to the heater at a level and duration such that both a transient change and a substantially steady-state temperature occur at the sensor. The thermal conductivity of the fluid of interest is determined based upon a known relation between the sensor output and the thermal conductivity at steady-state sensor temperatures. The specific heat and thermal diffusivity of the fluid of interest are determined based on a known relation among the thermal conductivity, the rate of change of the sensor output during a transient temperature change in the sensor, and the thermal diffusivity and specific heat.
A typical approach for determining the velocity of a fluid of interest is to determine the time require for a thermal wave to flow from a source heater element to a destination sensor element. By knowing the distance between the heater element and the sensor element, the velocity of the fluid can be calculated. This approach is suggested in U.S. Pat. No. 4,576,050 to Lambert. Lambert energizes a heater strip with an oscillating heater input signal to emit thermal waves in the fluid. The thermal waves propagate through the fluid at a rate that is dependent on the fluid velocity that flows perpendicular to the heater strip. A thermo-electric detector, spaced from one or both side of the heater, senses the thermal wave and provides a corresponding detector output signal. The velocity of the fluid is determined, at least to first order, from the time differential between the heater input signal and the detector output signal.
A limitation of many of the above prior art approaches is that a substantial amount of support hardware and/or software are required. For example, in many of the prior art approaches, a frequency generator is used to provide a frequency input signal to the heater element. Frequency generators can be relatively expensive, both in terms of hardware and power. Likewise, many of the prior art approaches require one or more high frequency timers to measure the time or phase lag between the heater input signal and a corresponding temperature disturbance in the fluid. Like fixed frequency generates, high frequency timers can be relatively expensive, both in terms of hardware and power.